The shift from gasoline vehicles to electric vehicles is increasingly important as measures for global environmental issues. For this reason, development of electric vehicles that use energy storage devices such as lithium ion secondary batteries as power sources is underway. Some of the energy storage devices include a separator of multilayer structure having a heat resistant coating layer or the like in order to ensure heat resistance of the separator.
Conventionally, energy storage devices have been proposed that include a multilayer structured separator capable of improving safety while suppressing reduction in battery performance (see, for example, Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-283273). In such an energy storage device, with the difference in air permeability of a plurality of layers constituting the separator satisfying a predetermined condition, safety can be improved while reduction in battery performance is suppressed.